An adder is used for obtaining an addition signal of a plurality of signals. FIG. 14 is a circuit diagram schematically illustrating an example of a conventional adder. In an adder 800 illustrated in FIG. 14, an operational amplifier 802 is virtually grounded to set a potential at a point A to 0 V. Accordingly, when input voltages V1 and V2 are input, a current I1 as expressed by Equation (1) below flows through a resistor Ra and a current I2 as expressed by Equation (2) below flows through a resistor Rb. Further, because input impedance of the operational amplifier 802 is so high that no current flows to the operational amplifier 802, a current If obtained by adding the current I1 and the current I2 as expressed by Equation (3) below flows through a feedback resistor Rf. In this case, the feedback resistor Rf, the current If, and an output voltage Vo of the adder 800 have the relationship of Equation (4) below. Then, from Equations (1) to (3) below, the output voltage Vo is determined by Equation (5) below. For example, if the resistors Ra and Rb and the feedback resistor Rf have the same resistance, the output voltage Vo is determined by Equation (6) below.I1=V1/Ra   (1)I2=V2/Rb   (2)If=I1+I2   (3)Vo=−Rf*If   (4)Vo=−Rf*((V1/Ra)+(V2/Rb))   (5)Vo=−(V1+V2)   (6)